A conventional low pressure metal vapor discharge lamp, as typically exemplified in a fluorescent lamp, has an elongated glass tube provided at both ends with electrodes and accommodating a rare gas of several torrs pressure and a small amount of a metal such as mercury. This type of lamp has a relatively long length which makes it inconvenient for many applications where a more compact light source is required.
Compact fluorescent lamps in the past have generally consisted of folded or bent glass tubing configured to obtain a desired arc length and lamp voltage. While the size of such packages is compatible with the size of an incandescent bulb, they require intricate bending of the lamp tube to achieve this, and in some cases a connecting "kiss joint" is required to join two glass tube sections together to obtain the desired arc length. These lamps, while highly efficient, are difficult and costly to manufacture.
In an attempt to satisfy the above demand, a lamp has been proposed which has a double-tube structure consisting of a fully closed outer glass bulb and an inner glass tube disposed within the outer glass bulb. The inner glass tube is open at one end but closed at the other end. One of the electrodes (cathode) is disposed within the inner glass tube adjacent the closed end, while the other electrode (anode) is disposed outside the inner glass tube. According to this arrangement, the discharge path formed between the two electrodes makes a turn at the open end of the inner glass tube, so that a sufficiently large length of the discharge path can be obtained with a relatively small overall length.
This known double-tube type of discharge lamp, however, has a problem in that it is difficult to uniformly distribute the discharge plasma over the entire discharging space between the inner glass tube and the outer glass bulb. More specifically, the discharge plasma outside the inner glass tube is concentrated locally to the region which exhibits the smallest resistance to the discharge current, and is not spread uniformly over the entire discharging space. It was believed that this local concentration of the discharge plasma could not be avoided even by the use of a ring-shaped anode disposed around the inner glass tube. In such a lamp, the luminous intensity is specifically high only at the region to which the plasma is locally concentrated, while only a low luminous intensity is obtained at portions of the lamp where the plasma is not distributed. Thus, it is difficult to obtain a uniform luminous intensity distribution over the entire lamp body.
In order to overcome the problem concerning the local concentration of the plasma in the known double-tube type fluorescent lamp, various solutions have been proposed. For example, U.S. Pat. No. 3,609,436 which issued to Campbell, proposes an improved lamp in which a plurality of anodes are disposed around the inner glass tube. These anodes are switched successively so as to forcibly rotate the locally concentrating plasma at a high speed around the inner glass tube to thereby achieve luminous intensity over the entirety of the lamp.
This improved lamp, however, requires a complicated and rather expensive transistor switching circuit for a high-speed switching of the voltage over the successive anodes and, therefore, is not practical from both technological and economical points of view.
Another proposed solution is described in U.S. Pat. No. 4,177,401, which issued to Yamane et al. This patent teaches a lamp wherein a single anode or alternatively a plurality of anodes is disposed around the inner glass tube. A permanent magnet is disposed near the open end of the inner glass tube for applying a magnetic field of a fixed intensity near the open end causing the discharge plasma to rotate about the axis of the discharge lamp.
In addition to requiring the added cost of the permanent magnet, the above lamp further requires that the outer glass bulb be modified to provide a tubular recess to accommodate the magnet.
Alternatively, U.S. Pat. No. 4,438,373, which issued to Watanabe et al, teaches notching the open end of the inner tube in alignment with a pair of anodes and using an inner glass tube having a non-circular cross-section.